Abstract

Photoabsorption above the first ionization potential of CO2 was observed at relatively low laser intensity, detected via resonant‐enhanced multiphoton ionization‐photoelectron spectra through several Rydberg states. This phenomenon can be explained by the presence of accidental resonances with long‐lived autoionizing states which make photon absorption within the ionization continuum possible. Laser powers are too low for this to be explained in terms of a ponderomotive potential and conventional above‐threshold ionization. This resonance‐enhanced above‐threshold absorption phenomenon is potentially useful in the study of excited and superexcited states. Photoelectron energies can be assigned to terminations on CO+2 ionic states at both the four‐ and five‐photon levels, allowing measurement of states up to 22 eV. Two unassigned bands may represent terminations on a new state of CO+2, with an ionization potential of 21.4 eV.